The C1q protein belongs to the C1 complex which initiates the conventional pathway of activation of the complement system which participates in innate immunity in mammals. In addition to C1q, the initiating complex comprises two serine proteases, C1r and C1s, associated as a C1s-C1r-C1r-C1s tetramer. The binding of C1q to “danger signals”, namely antigen-antibody complexes, in particular of the IgG type, and factors present at the surface of pathogenic agents, of infected cells or of apoptotic cells, results in the autoactivation of C1r, which activates C1s. Activated C1s then initiates the activation in cascade of the other complement components.
By virtue of its central role in the conventional complement activation pathway, a deficiency in C1q increases the sensitivity of the affected individual to microbial infections, such as Salmonella infections, malarial reinfection and polymicrobial peritonitis, but also causes excessive inflammation and autoimmunity of the lupus type, thereby indicating a tolerogenic or immunosuppressor role for C1q (Lu J. et al. (2008) Cellular & Molecular Immunology 5:9-21). As regards the latter disease, it has, moreover, been shown that patients suffering from systemic lupus erythematosus can be treated using an extracorporeal blood circulation system including a step of passing the blood over a C1q protein-based immunoadsorption column (Pfueller B. et al. (2001) Arthritis & Rheumatism 44:1962-1963).
Thus, it would be advantageous, in particular in a therapeutic or diagnostic context, or more broadly a scientific research context, to have safe and reproducible sources of C1q protein.
However, the structure of C1q makes complex the production thereof by synthesis. Indeed, C1q is a multimeric protein of high molecular weight (approximately 460 kDa) consisting of the association of 18 polypeptide chains: 6 C1qA subunits, 6 C1qB subunits and 6 C1qC subunits. Each subunit, or chain, contains a collagen-type N-terminal domain and a globular C-terminal domain. The C1qA subunit is associated with a C1qB subunit via the formation of a disulfide bridge and with a C1qC subunit noncovalently so as to form a heterotrimer. Moreover, the C1qC subunits are each connected in pairs via the formation of a disulfide bridge. For each heterotrimer, the collagen-type domains associate to form a triple helix which is also of collagen type, and the globular domains associate to form a virtually spherical globular region (Gaboriaud C. et al. (2003) Journal of Biological Chemistry 278:46974-46982). The overall shape of C1q is therefore that of a “bouquet of tulips” where the six globular regions emerge from a tail made up of the six collagen-type triple helices (FIG. 1).
Consequently, at the current time, the only means of obtaining complete C1q protein is to extract it from human or animal serum, which is satisfactory neither in terms of reproducibility, nor in terms of biological safety, since blood derivatives may be contaminated with viruses, prions or else single-cell parasites. As regards the recombinant route, which would make it possible to solve these problems, it has been possible to produce only subunits or fragments, in particular globular ones (see, for example, Kojouharova M. et al. (2004) Journal of Immunology 172:4351-4358), of C1q.
It is therefore an object of the invention to provide a reproducible method for obtaining complete and correctly structured C1q protein without associated biological risk.